We have developed an in vivo culture system in which hematopoietic cell suspensions are immobilized in a plasma clot within a diffusion chamber implanted into the peritoneal cavity of a host animal. Committed stem cells grow into colonies which can be readily counted and characterized. In the rat, normal bone marrow suspensions show peak erythrocytic colony formation on day 2 and subsequently on day 7-8 following implant. The secondary growth wave appears similar to a burst formation. Granulocytic colonies peak on day 4 following implant. Anemic host animals augment erythroid colony growth. Endotoxin treated hosts depress erythroid colony growth while augmenting granulocytic colony size. Normal bone marrow granulocytic colony formation is inhibited by host animals with acute myelogenous leukemia (Shay Chloroleukemia). Normal mouse bone marrow and spleen cell suspensions are capable of hemic cell colony growth. The plasma clot-diffusion chamber culture system is a useful procedure for assessing the functional capabilities of committed stem cells. The host animal provides a physiological environment with the nutritive and humoral requisites for growth of isolated hematopoietic cell suspensions. The system allows for perturbation of both donor and host animal so that within an experimental design the proliferative and differentiative abilities of committed precursor elements can be determined.